Why cant cities skylines generate terrain like how minecraft does it? Wont we have more than 81 tiles this way?
Try having 33M × 33M blocks loaded at the same time and your PC will explode. The way this works is that the game unloads chunks that are far away - not rendering, not simulating.
If you would do this to Cities: Skylines, it would create more problems than solutions. You create an industrial area somewhere, move somewhere else, and suddenly all shops are without goods. Why? Because the industrial area is unloaded and therefore not sending trucks!
About more vanilla tiles... in recent requests for more tiles (go just one or two pages back), there was a clear answer why they aren't added. As the devs said in a livestream, consoles can barely handle 9 filled tiles, 25 would turn them into a fireplace. And if it would be added only to PC, then
1) most request are from console players, so problem not solved
2) it would create a massive imbalance between PC and console
And about increasing 81 to 100, some computers only handle vanilla 9 tiles, most can handle 25, but only strong computers can handle a fully developed, 81 tile city. So with 100 tiles, you can turn your computer into a volcano-in-a-box